A cellular telecommunications system is based around cells or similar radio coverage areas. Examples of cellular telecommunications systems include standards such as, without limiting to these, AMPS (American Mobile Phone System), DAMPS (Digital AMPS), GSM (Global System for Mobile communications) or various GSM based systems (such as GPRS: General Packet Radio Service), CDMA (Code Division Multiple Access) or the proposed WCDMA (Wideband CDMA), or UMTS (Universal Mobile Telecommunications System) etc. In general, a cell coverage area or radio transceiver station coverage area of a telecommunications system can be defined as a certain geographically limited area covered by one or several base transceiver stations (BTS) serving mobile stations (MS) via an air or radio interface and usually connected to a base station subsystem (BSS). Each of the coverage areas of the system can be controlled by an appropriate controller apparatus. For example, in the currently proposed WCDMA standard each of the cells is controlled by at least one radio network controller (RNC), in the GSM system each of the cells is controlled by a base station controller (BSC) controlled by a mobile switching center (MSC) and in the GPRS system each of the cells is controlled by a serving GPRS support node (SGSN). The controller is connected further to a gateway or linking apparatus, such as a gateway GPRS support node (GGSN) or gateway mobile switching center (GSMC), linking the cell to the other parts of the communication system, such as to a PSTN (Public Switched telecommunications Network) or to a data network, such as to a X.25 based network or to a TCP/IP (Transmission Control Protocol/Internet Protocol) based network. Several cells connected to each other cover a larger geographical area, and thus several cells form together the coverage area of a cellular telecommunications network.
The user equipment (UE), such as a mobile station (MS), within one of the cells of the telecommunications system is respectively controlled by the controller functionality of the given cell. The UE may be controlled by only one controller at time. However, the UE may also be connected simultaneously to several controllers. This may occur e.g. when the cells overlap or in so called soft handoff mode, where the UE may be in communication with two base stations and those base stations may be connected to different controllers, or when one controller is controlling another controller controlling the UE. One controller can be defined as the serving (main) controller whereas the others act as secondary controllers.
The cellular telecommunications systems comprise further a registration functionality in order to be able to control the movements and operation of the mobile stations subscribing to the cellular telecommunications network. Each station subscribing to a cellular telecommunications network is permanently registered in the network system which stores subscriber related data. This data may be the current location of the mobile station so that continuous track of the location can be kept for the purposes of call routing. In the GSM terminology, the permanent location registers are often referred to as home location registers (HLR). Each network may comprise one or several permanent or home location register. The register can be a stand-alone network element or the register can be built into a network controller or into any other network element.
In addition to the permanent registers, the cellular telecommunications networks comprise temporary or visitor location (VLR) registers. These temporary registers handle only temporary i.e. visiting subscribers, i.e. the mobile stations within the service area of the associated network controller at a given point of time. The temporary location register keeps track of the given service area within which the mobile station may be located and is constantly informed of whether the mobile station is ON or OFF. Each network controller may have its own visitor location register or the network controllers may share a stand-alone visitor location register or a visitor location register disposed within another network element.
The user equipment or mobile station communicates with an appropriate network controller and provides the controller with various kinds of information. For instance, in a ‘cell update’ or similar situation, e.g. when the mobile station (MS) changes from one cell to a new cell or when the mobile station is switched on in one of the cells after having been switched off or unreachable for a while, the mobile station will send a message containing a MS identifier (ID) to the controller of the new cell. The controller of the new cell may be the same as in the previous cell or it may be different from the previous controller handling the MS before the cell change. The MS ID comprises data required by the telecommunications system in order to be able to handle the incoming/outgoing call and signalling to/from the MS, respectively. Once the MS ID is received by the system, the system is aware of the current location of the particular MS.
The use of cells or similar geographically restricted and known radio coverage areas and associated controllers facilitates the telecommunications system to produce at least a rough location estimate concerning the current location of an individual mobile station. More particularly, the telecommunications system is always aware (at least roughly) of the current location of such mobile stations which are communicating with at least one of the base stations of the system and are thus registered within at least one of the location registers of the system (i.e. are located within one limited radio coverage area of the system). This information is available even when the mobile station is located within a coverage area of a visited or “foreign” network, as the visited network is capable of transmitting the location of the mobile station back to the home location register, e.g. for the purposes of routing and charging. It is also possible to input location information provided by an external system for providing location information of a station into the telecommunications system, and to subsequently process and utilize this location information in the telecommunications system.
There is a proposal for a location service feature provided by means of a cellular telecommunications network which could provide the last known location of a mobile station together with a time-stamp. This feature can be provided by a separate network element or server which receives the information from the various controllers of the system. More particularly, the determination of the location of the mobile user can be based on the location information received from databases of the visited controllers of the telecommunications system, e.g. from a visitor location register (VLR) in the GSM system. This type of arrangement would give the location to within of one base station or cell, i.e. it would indicate that the mobile station is (or at least was) within the coverage area of a certain base station or cell. The accuracy of the location determination can be improved by e.g. utilizing results of measurements which define the travel time (or travel time differences) of the radio signal sent by the mobile station to the base station.
However, this approach could cause a relatively large amount of signalling load to the telecommunications system. When the location server sends an enquiry for location information, the request will be directed firstly to the home location register. The home location register, however, does not have the location information and may thus only give the address of a correct visitor location register and/or reroute the enquiry to a correct visitor location register including the requested data. The enquiry is thus redirected from the home location register to the visitor location register, and only after this the required data can be retrieved and transmitted as the response to the enquiry to the location server. The location information could also be stored in the home location register, but this would cause too much signalling between the visitor location register and the home location register. This is because each of the visitor location registers would then be continuously sending location information to the home location register every time a mobile station moves such that the location information needs an update. As a large number of mobile stations subscribe to each of the mobile telecommunications networks, this would cause a greater signalling load than the network system could handle. This extra transmission load would have an adverse effect on the operation of the communications system and would consume more of the limited network resources.
In addition, the processing and storing of the geographical location information requires additional memory resources in the communication system. The various location registers of the communication system may have only limited capabilities to store any additional information. This is especially the case if a location update history for a predefined period (e.g. for the last hour or day) is to be collected and stored. In case the history information is used, it is possible that the station has moved a relatively long way, whereby the “too old” information becomes irrelevant and may even cause an error in the location estimate. In case only the last determined geographical location of the user equipment is stored and used for the location estimation, this may not in all instances give the most reliable results. This kind of approach may even give a seriously incorrect location, e.g. in a situation where a emergency call becomes incorrectly located into a wrong side of a river.